Normally, it is safe to upgrade from 2.3.X to 2.5.1 as there is not much of change in the way Plugins and themes interact. I have updated around 10 – 15 separate WordPress installations (including some client blogs) and have not faced any issue.

But it also depends on the Plugins that you are using and the amount of customization you have done to your theme. I would recommended you to check whether the Plugins that you are using are compatible with 2.5.1.

Hey,
I upgraded to 2.5.1 finally… there’re minor issues with Brian’s threaded comment plugin otherwise looks okay. If you dont’ mind could you take a real quick look at it and see if there’s any ‘visible damage’?